Interface trap charge density reduction

ABSTRACT

The present disclosure provides methods of fabricating a semiconductor device. A method according to one embodiment includes forming, on a substrate, a first fin formed of a first semiconductor material and a second fin formed of a second semiconductor material different from the first semiconductor material, forming a semiconductor cap layer over the first fin and the second fin, and annealing the semiconductor cap layer at a first temperature while at least a portion of the semiconductor cap layer is exposed.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No,62/906,291, filed Sep. 26, 2019, the entirety of which is incorporatedby reference herein

BACKGROUND

The semiconductor integrated circuit (IC) industry has experienced rapidgrowth. In the course of IC evolution, functional density (i.e., thenumber of interconnected devices per chip area) has generally increasedwhile geometry size (i.e., the smallest component (or line) that can becreated using a fabrication process) has decreased. This scaling downprocess generally provides benefits by increasing production efficiencyand lowering associated costs. However, such scaling down has also beenaccompanied by increased complexity in design and manufacturing ofdevices incorporating these ICs, and, for these advances to be realized,similar developments in device fabrication are needed.

As geometry size of IC devices continues to shrink, defects that mayhave been largely inconsequential to performance of conventional largerdevices may now substantially impact device performance. For example,p-type fully-strained channel technology has been developed to improvehole mobility in p-type transistors. A silicon cap may be formed over ap-type fully-strained channel protect undesirable oxidation ofsemiconductor material in the p-type channel. It has been observed thata high density of interface trap charge may be present at the interfacebetween the silicon cap and the p-type fully-strained channel, resultingin current leakage and increase in channel resistance. Therefore,although conventional p-type fully-strained channel devices are adequatefor their intended purposes, they are not satisfactory in all aspects.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are riot drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a flow chart of a method for fabricating a semiconductordevice on a workpiece, according to aspects of the present disclosure.

FIGS. 2-12 are fragmentary cross-sectional views of the workpiece atvarious steps of the method of FIG. 1 , according to aspects of thepresent disclosure.

DETAILED DESCRIPTION

It is understood that the following disclosure provides many differentembodiments, or examples, for implementing different features of variousembodiments. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Forexample, the formation of a first feature over or on a second feature inthe description that follows may include embodiments in which the firstand second features are formed in direct contact, and may also includeembodiments in which additional features may be formed interposing thefirst and second features, such that the first and second features maynot be in direct contact. In addition, the present disclosure may repeatreference numerals and/or letters in the various examples. Thisrepetition is for the purpose of simplicity and clarity and does not initself dictate a relationship between the various embodiments and/orconfigurations discussed.

Moreover, the formation of a feature on, connected to, and/or coupled toanother feature in the present disclosure that follows may includeembodiments in which the features are formed in direct contact, and mayalso include embodiments in which additional features may be formedinterposing the features, such that the features may not be in directcontact. In addition, spatially relative terms, for example, “lower,”“upper,” “horizontal,” “vertical,” “above,” “over,” “below,” “beneath,”“up,” “down,” “top,” “bottom,” etc., as well as derivatives thereof(e.g., “horizontally,” “downwardly,” “upwardly,” etc.) are used for easeof the present disclosure of one features relationship to anotherfeature. The spatially relative terms are intended to cover differentorientations of the device including the features. Still further, when anumber or a range of numbers is described with “about,” “approximate,”and the like, the term is intended to encompass numbers that are withina reasonable range including the number described, such as within +/−10%of the number described or other values as understood by person skilledin the art. For example, the term “about 5 nm” encompasses the dimensionrange from 4.5 nm to 5.5 nm.

As IC devices shrink in size, short channel effect (SCE) has preventedfurther scaling down of planar field effect transistors (FETs). Variousmulti-gate devices have been proposed to enhance gate control andprevent short channel effect. An example of these multi-gate devices isa fin-shape field effect transistor (FinFET). Further improvements ofFinFETs have been put forth over the years. For instances, mobility ofcharge carriers (i.e. electrons in n-type devices and holes in p-typedevices) may be increased by forming FinFETs with strained channels. Thematerials used in strained channels can be different for p-type fieldeffect transistors (PFETs) and n-type field effect transistors (NFETs).By way of example and not limitation, electron mobility in an NFET canbe enhanced with the use of fully strained silicon/carbon-doped silicon(Si/Si:C) channels, while hole mobility in PFETs can be enhanced withfully strained silicon germanium (SiGe) channels. Fully strainedsilicon/carbon-doped silicon (Si/Si:C) channels and fully strainedsilicon germanium (SiGe) channels may be formed using epitaxial growth.

The fully strained epitaxial channels can be formed from epitaxiallayers disposed on a top portion of a silicon (Si) fin. The formationprocess of fully strained channels requires numerous photolithography,etch, pre-treatment, anneal and growth operations. To further strain thep-type channel and to prevent germanium from undesirable oxidation, asilicon cap layer may be formed over workpiece to cover the fullystrained p-type channel. It has been observed that properties of thesilicon cap layer affect performance of the semiconductor device. Whenthe silicon cap layer lacks crystallinity, the silicon cap layer mayoxidize too quickly and may not adequately prevent germanium content inthe p-type channel from oxidation. In addition, when silicon cap layercontains defects at its interface with the p-type channel, the densityof interface trap charge (DIT) may increase, resulting in leakagecurrent, reduction in carrier mobility, and increase in channelresistance. A p-type fully strained channel (PFSC) can be susceptible todefects where the Si to SiGe lattice mismatch is larger, for example,compared to an n-type fully strained channel (NFSC) where Si, Si:C, or acombination thereof can be used.

FIG. 1 illustrate a flow chart of a method 100 for fabricating asemiconductor device on a workpiece 200. FIG. 1 will be described inconjunction with FIGS. 2-12 , which illustrate fragmentarycross-sectional views of the workpiece 200 at various stages offabrication according to various embodiments of method 100 of thepresent disclosure. Additional steps can be provided before, during, andafter method 100, and some of the steps described can be moved,replaced, or eliminated for additional embodiments of method 100.Additional features can be added in the workpiece 200 depicted in FIGS.2-12 , and some of the features described below can be replaced,modified, or eliminated in other embodiments of the workpiece 200depicted in FIGS. 2-12 . Because a semiconductor device will be formedfrom the workpiece 200 after the fabrication process concludes, theworkpiece 200 may also be referred to as the semiconductor device 200from time to time throughout the present disclosure for convenience. Thesemiconductor device 200 can be included in a microprocessor, a memory,and/or other IC device. In some implementations, the semiconductordevice 200 is a portion of an IC chip, a system on chip (SoC), orportion thereof, that includes various passive and activemicroelectronic devices, such as resistors, capacitors, inductors,diodes, p-type field effect transistors (PFETs), n-type field effecttransistors (NFETs), metal-oxide semiconductor field effect transistors(MOSFETs), complementary metal-oxide semiconductor (CMOS) transistors,bipolar junction transistors (BJTs), laterally diffused MOS (LDMOS)transistors, high voltage transistors, high frequency transistors, othersuitable components, or combinations thereof.

Referring to FIGS. 1, 2 and 3 , the method 100 includes a block 102where a first fin 216A of a first semiconductor material 206 and asecond fin 218A of a second semiconductor material 204 over a substrate202. Reference is first made to FIG. 2 , which illustrates the substrate202 that includes an n-type well (n-well) 202N and a p-type well(p-well) 202P. The substrate 202 may include silicon. Alternatively oradditionally, substrate 202 includes another elementary semiconductor,such as germanium; a compound semiconductor, such as silicon carbide,gallium arsenide, gallium phosphide, indium phosphide, indium arsenide,and/or indium antimonide; an alloy semiconductor, such as silicongermanium (SiGe), GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP;or combinations thereof. In some implementations, substrate 202 includesone or more group III-V materials, one or more group II-IV materials, orcombinations thereof. In some implementations, substrate 202 is asemiconductor-on-insulator substrate, such as a silicon-on-insulator(SOI) substrate, a silicon germanium-on-insulator (SGOI) substrate, or agermanium-on-insulator (GOI) substrate. Semiconductor-on-insulatorsubstrates can be fabricated using separation by implantation of oxygen(SIMOX), wafer bonding, and/or other suitable methods. In the embodimentdepicted in FIG. 2 , substrate 202 consists essentially of silicon. Then-well 202N include n-type dopants, such as phosphorus, arsenic, othern-type dopant, or combinations thereof. The p-well 202P include p-typedopants, such as boron, indium, other p-type dopant, or combinationsthereof. The n-well 202N and p-well 202P can be formed directly onand/or in substrate 202 by ion implantation processes, diffusionprocesses, and/or other suitable doping process.

After the n-well 202N and the p-well 202P are formed, a layer of thefirst semiconductor material 206 and a layer of the second semiconductormaterial 204 are formed over the workpiece 200. In some embodiments, alayer of the second semiconductor material 204 is first epitaxiallygrown on the substrate 202, a portion of the layer of the secondsemiconductor material 204 is removed to form a recess, and then a layerof the first semiconductor material 206 is epitaxially grown to fill therecess. In some implementations, the layer of the second semiconductormaterial 204 is not directly grown from the substrate 202. In thoseimplementations, a seed layer (not shown) may be formed on the substrate202 and the layer of the second semiconductor material 204 is thenformed on the seed layer. In some embodiments, the second semiconductormaterial 204 may include silicon, carbon, or a combination thereof. Insome embodiments, the first semiconductor material 206 may includesilicon, germanium, or a combination thereof. In some instances wherethe first semiconductor material 206 includes silicon and germanium, agermanium content of the first semiconductor material 206 may be between20% and 80%. An example where the second semiconductor material 204 isformed of silicon is described below for illustration purposes. Thelayer of the second semiconductor material 204 is epitaxially grown to athickness between about 50 nm to about 100 nm using source gases such assilane (SiH₄), silicon tetrachloride (SiCl₄), trichlorosilane (TCS), ordichlorosilane (SiH₂Cl₂ or DCS). Hydrogen (H₂) can be used as a reactantgas that reduces the aforementioned source gases. The depositiontemperature during the epitaxial layer formation can range from about700° C. to about 1250° C. depending on the gases used. For example,source gases with fewer chlorine atoms (e.g., DCS) may require lowerformation temperatures compared to source gases with more chlorineatoms, such as SiCl₄ or TCS. The aforementioned ranges and type of gasesare provided as examples and are not limiting. To form a recess in thelayer of the second semiconductor material 204, a hard mask formed of adielectric material may be formed thereover. The hard mask may be formedof silicon oxide or silicon nitride. A photolithography process is thenperformed to pattern the hard mask. An etch process is then performedusing the patterned hard mask as an etch mask to etch the recess in thelayer of the second semiconductor material 204. The etch process may bea dry etch process or a wet etch process with suitable etchingchemistry. In some instances, the etch process forming the recess doesnot remove all of the second semiconductor material 204 at the bottom ofthe recess such that a thickness of the second semiconductor material204 at the bottom may serve as a seed layer for the first semiconductormaterial 206 that fills the recess. A layer of the first semiconductormaterial 206 is then formed to fill the recess in the layer of thesecond semiconductor material 204. According to some embodiments, thelayer of the first semiconductor material 206 is epitaxially grown at atemperature between about 550° C. to about 700° C. using precursor gasessuch as SiH₄, Si₂H₆, SiH₂Cl₂, GeH₄, HCl, or a combination thereof, andreactant gases such as H2, N2, or Ar, or a combination thereof.

After the first semiconductor material 206 is epitaxially grown to fillthe recess in the layer of the second semiconductor material 204, aplanarization process, such as a chemical mechanical polishing (CMP),may be performed to planarize the top surface of the first semiconductormaterial 206 and the second semiconductor material 204 such that theyare coplanar. In some embodiments, after the planarization process, thefirst semiconductor material 206 has a thickness between about 45 nm andabout 70 nm. Thereafter, silicon top layer 208 is epitaxially grown to athickness between about 1.5 nm and about 5 nm over the planarized topsurfaces of the first semiconductor material 206 and the secondsemiconductor material 204. In some implementations, the as depositedsilicon top layer 208 may be trimmed by etching back or polishing to areduced thickness between about 0.5 nm and about 4 nm.

To form the first fin 216A and the second fin 218A shown in FIG. 3 , ahard mask 211 (FIG. 2 ) may be deposited over the silicon top layer 208.The hard mask 211 may be a single layer or a multilayer. In theembodiments represented in FIG. 2 , the hard mask 211 is a multilayerthat includes a first hard mask layer 210 and a second hard mask layer212. The first hard mask layer 210 and the second hard mask layer 212have different compositions. In some embodiments, the first hard masklayer 210 is formed of silicon oxide and the second hard mask layer 212is formed of silicon nitride. In some implementations, a combination ofdeposition, lithography and/or etching processes are performed to definethe first fin 216A and the second fin 218A over the substrate 202. Forexample, forming the first fin 216A and the second fin 218A includesperforming a lithography process to form a patterned resist layer overthe hard mask 211 and performing an etching process to transfer apattern defined in the patterned resist layer to the hard mask 211. Thelithography process can include forming a resist layer on the hard mask211, performing a pre-exposure baking process, performing an exposureprocess using a mask, performing a post-exposure baking process, andperforming a developing process. During the exposure process, the resistlayer is exposed to radiation energy (such as ultraviolet (UV) light,deep UV (DUV) light, or extreme UV (EUV) light), where the mask blocks,transmits, and/or reflects radiation to the resist layer depending on amask pattern of the mask and/or mask type (for example, binary mask,phase shift mask, or EUV mask), such that an image is projected onto theresist layer that corresponds with the mask pattern. Since the resistlayer is sensitive to radiation energy, exposed portions of the resistlayer chemically change, and exposed (or non-exposed) portions of theresist layer are dissolved during the developing process depending oncharacteristics of the resist layer and characteristics of a developingsolution used in the developing process. After development, thepatterned resist layer includes a resist pattern that corresponds withthe mask. The etching process uses the patterned resist layer as an etchmask to remove portions of the hard mask 211. The etching process caninclude a dry etching process (for example, a reactive ion etching (RIE)process), a wet etching process, other suitable etching process, orcombinations thereof. After the etching process, the patterned resistlayer is removed from substrate 202, for example, by a resist strippingprocess. Alternatively, first fin 216A and the second fin 218A areformed by a multiple patterning process, such as a double patterninglithography (DPL) process (for example, alithography-etch-lithography-etch (LELE) process, a self-aligned doublepatterning (SADP) process, a spacer-is-dielectric (SID) SADP process,other double patterning process, or combinations thereof), a triplepatterning process (for example, alithography-etch-lithography-etch-lithography-etch (LELELE) process, aself-aligned triple patterning (SATP) process, other triple patterningprocess, or combinations thereof), other multiple patterning process(for example, self-aligned quadruple patterning (SAQP) process), orcombinations thereof. In some implementations, directed self-assembly(DSA) techniques are implemented while forming first fin 216A and thesecond fin 218A. Further, in some implementations, the exposure processcan implement maskless lithography, electron-beam (e-beam) writing,ion-beam writing, and/or nanoimprint technology for patterning theresist layer and/or other layers.

In some embodiments represented in FIG. 3 , the semiconductor device 200includes double-fin transistors and two fins are formed over each of then-well 202N and the p-well 202P. In these embodiments, the first fin216A and an adjacent fin 216B are formed over the n-well 202N and thesecond fin 218A and an adjacent fin 218B are formed over the p-well202P. The first fin 216A and the adjacent fin 216B may be collectivelyreferred to as p-type fins 216 and the second fin 218A and the adjacentfin 218B may be collectively referred to as n-type fins 218. The p-typefins 216 serve as p-type active regions for formation of p-typetransistors with p-type fully-strained channels. The n-type fins 218serve as n-type active regions for formation of n-type transistors withn-type channels or strained n-type channels. However, the presentdisclosure is not to be construed restrictively. With reference to thepresent disclosure, a person of ordinary skill in the art wouldappreciate that the methods and devices disclosed in the presentdisclosure are readily applicable to semiconductor devices that includesingle-fin or multi-fin (i.e., having more than 3 fins per transistor)transistors.

Referring now to FIGS. 1 and 4 , the method 100 includes a block 104where a liner 214 is formed over the first fin 216A and the second fin218A. The liner 214 serves to provide structural support to the firstfin 216A and the second fin 218A during subsequent processes. In someembodiments, the liner 214 may be formed of silicon nitride to athickness between about 0.5 nm and about 3 nm. In some implementations,the liner 214 may be formed using atomic layer deposition (ALD) orchemical vapor deposition (CVD).

Referring to FIGS. 1, 5 and 6 , the method 100 includes a block 106where an isolation feature 220 between the first fin 216A and the secondfin 218A. In some embodiments represented in FIG. 5 , a dielectricmaterial 219 is first blanketly deposited over the workpiece 200 to fillspace between the p-type fins 216 and the n-type fins 218. Thedielectric material 219 may include silicon oxide, silicon oxynitride,other suitable isolation material, or combinations thereof and may bedeposited by chemical vapor deposition (CVD) or a spin-on-glass (SOG)process. A planarization process, such as a CMP process, is thenperformed until the hard mask layer 211 is removed. Referring now toFIG. 6 , the planarized dielectric material 219 is then etched back toform the isolation structure 220. As shown in FIG. 6 , the firstsemiconductor material 206 and the second semiconductor material 204 inthe first fin 216A and the second semiconductor material 204 in thesecond fin 218A are exposed and rise above the isolation structure 220.In embodiments represented in FIG. 6 , the isolation structure 220 is ashallow trench isolation (STI) structure and may be referred to as STIstructure 220. However, the present disclosure envisions implementationsof other types of isolation structure, such as deep trench isolation(DTI) structures and local oxidation of silicon (LOCOS) structures, solong as such implementation does not hinder formation of semiconductordevices with strained channels. In some embodiments, the etching back ofthe dielectric material 219 also removes the liner 214 withoutsubstantially etching the isolation structure 220, the firstsemiconductor material 206 and the second semiconductor material 204. Insome other embodiments, a separate etch process is performed toselectively remove the liner 214. In embodiments where the liner 214 isformed of silicon nitride, the separate etch process is configured toselectively etch silicon nitride.

Referring to FIGS. 1 and 7 , the method 100 includes a block 108 where asilicon cap layer 222 is formed over the first fin 216A and the secondfin 218A, The silicon cap layer 222 may be deposited using suitabledeposition techniques, such has atomic layer deposition (ALD) orepitaxial growth. In some implementations, the silicon cap layer 222 maybe epitaxially grown to a thickness between about 0.5 nm and about 5 nmusing source gases such as silane (SiH₄), silicon tetrachloride (SiCl₄),trichlorosilane (TCS), or dichlorosilane (SiH₂Cl₂ or DCS). Hydrogen (H₂)can be used as a reactant gas that reduces the aforementioned sourcegases. The deposition temperature during the epitaxial layer formationcan range from about 380° C. to about 1250° C. depending on the gasesused. For example, source gases with fewer chlorine atoms (e.g., DCS)may require lower formation temperatures compared to source gases withmore chlorine atoms, such as SiCl₄ or TCS. In some otherimplementations, the silicon cap layer 222 may be formed using ALD to asimilar thickness range between about 0.5 nm and about 5 nm usingsimilar gas precursors. The difference between ALD and epitaxial grownlies primarily in that alternating self-limiting monolayers are formedin the former and the same are not necessarily formed in the latter.Other differences may include reactor types and process conditions. Insome embodiments, the silicon cap layer 222 is first formed to a firstthickness and then recessed to a smaller second thickness that fallswith the thickness range between about 0.5 nm and about 5 nm.

Referring to FIGS. 1 and 8 , the method 100 includes a block 110 wherethe silicon cap layer 222 undergoes an anneal process 300 to crystallizethe silicon cap layer 222. In some embodiments, the anneal process 300at block 110 is performed using radiation heating at a first temperature(T1) between about 800° C. and about 1050 C. The disclosed temperaturerange is critical to the anneal process 300 at block 110. When the firsttemperature (T1) of the anneal process 300 is below 800° C., little orno reflow of silicon may take place to improve crystallinity of thesilicon cap layer 222 and to reduce density of interfacial trap chargebetween the silicon cap layer 222 and the first semiconductor layer 206.When the first temperature (T1) of the anneal process 300 is above 1050°C., the p-type fins 216 (including the first fin 216A) and n-type fins218 (including the second fin 218A) may be damaged. The anneal process300 at block 110 may be performed at a first pressure (P1) between about0.001 atmosphere (atm) and about 1.1 atm. It has been observed that afinite amount of oxygen may be needed when the anneal pressure of theanneal process 300 is well below 1 atm, such as at 0.02 atm, to controlreflow of the silicon cap layer 222. As both low pressure and lack ofoxygen would increase silicon diffusivity in the silicon cap layer 222,the finite amount of oxygen may prevent too much silicon from reflowthat may result in non-uniform thickness in the silicon cap layer 222.Such a finite amount of oxygen is not needed when the first pressure(P1) of the anneal process 300 is at or around 1 atm, such as at 1.02atm. The anneal process 300 at block 110 may be performed in a gasambient that includes only nitrogen, substantially free of oxygen. Whenthe first pressure (P1) at block 110 is well below 1 atm, the gasambient includes nitrogen and oxygen, with an oxygen content at betweenabout 0.01% and about 0.05%, such as at about 0.025%. When the firstpressure (P1) of the anneal process 300 is at or around 1 atm, the gasambient include nitrogen only. In some instances, more than one purgingoperation may be needed to intentionally remove oxygen gas from the gasambient when the first pressure (P1) of the anneal process 300 is at oraround 1 atm. Because the anneal process 300 is performed afterformation of the silicon cap layer 222, the anneal process 300 may bereferred to as the post-silicon-cap anneal (PSA) 300. It has beenobserved that PSA process 300 may effectively reduce the density ofinterface trap charge (DIT) at the interface between silicon cap layer222 and the first semiconductor material 206 and to improvecrystallinity of the silicon cap layer 222. Transmission electronmicroscope (TEM) images show that a more clear-cut interface between thesilicon cap layer 222 and the first semiconductor material 206 may beformed as a result of the PSA process 300. The DIT is reduced becauselattice defects and dislocation at the interface are reduced in the PSAat block 110. Because the rate of oxidation of silicon decreases withcrystallinity, the improved crystallinity of the silicon cap layer 222slows down the oxidation rate and provides the first semiconductor layer206 with better protection from undesirable oxidation.

Because the PSA at block 110 is performed before formation ofsource/drain features, metal gate stacks, and other metal-containingcontact features, the relatively high first temperature T1 (i.e.,between about 800° C. and about 1050° C.) of PSA does not come with anyrisk of exceeding thermal budget or causing thermal damages to theaforementioned structures.

Referring to FIGS. 1 and 9 , the method 100 includes a block 112 wherean interfacial layer 224 is deposited over the silicon cap layer 222. Insome embodiments, the interfacial layer 224 includes silicon oxide andis deposited on the silicon cap layer 222 using a suitable depositionprocess, such as chemical vapor deposition (CVD) or atomic layerdeposition (ALD). In some implementations, an anneal process may beperformed to anneal the deposited interfacial layer 224 to reduceimperfections and improve quality of the interfacial layer 224.

Referring now to FIGS. 1 and 10 , the method 100 includes a block 114where source/drain features 226, 228 and a metal gate stack 234 areformed. The metal gate stack 234 may be formed using a gate firstprocess or a gate last process. The latter is described below as anexample and the former is fully envisioned. In a gate last process, adummy gate stack (not shown) may be first formed over the p-type fins216 and the n-type fins 218 such that the dummy gate stack wraps aroundeach of the p-type fins 216 and the n-type fins 218 and engage its topsurface and side surfaces. In some instances, the dummy gate stack mayinclude polysilicon and may be formed on the interfacial layer 224(illustrated in FIG. 9 ). After the dummy gate stack is formed, a gatespacer 230 may be deposited over the dummy gate stack. Thereafter, adielectric layer (not shown) is deposited over the workpiece 200 tocover the dummy gate stack, the p-type fins 216 and the n-type fins 218and then planarized to form a level top surface. In some embodiments,photolithography processes may be then performed to sequentially exposeand recess source/drain regions of the p-type fins 216 and the n-typefins 218 to form p-type source/drain features 226 and n-typesource/drain feature 228. Epitaxial source/drain features may then bedisposed in recessed source/drain regions of the p-type fins 216 and then-type fins 218. An epitaxy process can implement CVD depositiontechniques (for example, vapor-phase epitaxy (VPE), ultra-high vacuumCVD (UHV-CVD), LPCVD, and/or PECVD), molecular beam epitaxy, othersuitable SEG processes, or combinations thereof. In someimplementations, P-type epitaxial source/drain feature 226 may be formedof silicon germanium doped with p-type dopants, such boron. N-typeepitaxial source/drain features 228 may be formed of silicon or Si:Cdoped with n-type dopants, such as phosphorus. In some implementations,annealing processes are performed to activate dopants in epitaxialsource/drain features 226 and 228 of the semiconductor device 200.According to some implementations, after the epitaxial source/drainfeatures 226 and 228 are deposited, a contact etch stop layer (CESL, notshown) may be deposited over the p-type source/drain feature 226 and then-type source/drain feature 228. The CESL layer may be formed of siliconnitride or silicon carbonitride. Thereafter, an interlayer dielectriclayer (ILD) 232 may be deposited over the workpiece 200 to cover thep-type source/drain features 226 and n-type source/drain feature 228.The ILD 232 may be formed of silicon oxide, silicon nitride, siliconoxynitride, TEOS formed oxide, PSG, BPSG, low-k dielectric material,other suitable dielectric material, or combinations thereof. Exemplarylow-k dielectric materials include FSG, carbon doped silicon oxide,Black Diamond® (Applied Materials of Santa Clara, Calif.), Xerogel,Aerogel, amorphous fluorinated carbon, Parylene, BCB, SiLK (DowChemical, Midland, Mich.), polyimide, other low-k dielectric material,or combinations thereof. In some implementations, the ILD 232 may beformed by flowable CVD (FCVD) or spin-on coating. For ease ofillustration, ILD 232 is made transparent in FIG. 10 to show the gatespacer 230.

The ILD 232 is planarized using a suitable process such a CMP processuntil the dummy gate stack is exposed. In the example gate last processdescribed herein, gate replacement operations are then performed toreplace the dummy gate with a metal gate stack 234. First, the dummygate stack and/or a portion of the interfacial layer 224 is removed toform a gate trench. Then the metal gate stack 234 is deposited in thegate trench. In some embodiments, the metal gate stack 234 may include agate dielectric layer and a gate electrode over the gate dielectriclayer. In some implementations, the gate dielectric layer may include asilicon oxide layer and a high-k dielectric layer. The silicon oxidelayer may be the leftover interfacial layer 224 or may be formed anewwhen all or a substantial portion of the interfacial layer 224 isremoved. The high-k dielectric layer is formed of a dielectric materialhaving a high dielectric constant, for example, greater than adielectric constant of silicon oxide (k≈3.9). Exemplary high-kdielectric materials include hafnium, aluminum, zirconium, lanthanum,tantalum, titanium, yttrium, oxygen, nitrogen, other suitableconstituent, or combinations thereof. In some implementations, thehigh-k dielectric layer may include, for example, HfO₂, HfSiO, HfSiON,HfTaO, HfTiO, HfZrO, ZrO₂, Al₂O₃, HfO₂—Al₂O₃, TiO₂, Ta₂O₅, La₂O₃, Y₂O₃,other suitable high-k dielectric material, or combinations thereof, Thegate electrode includes an electrically conductive material. In someimplementations, the gate electrode includes multiple layers, such asone or more capping layers, work function layers, glue/barrier layers,and/or metal fill (or bulk) layers. A capping layer can include amaterial that prevents or eliminates diffusion and/or reaction ofconstituents between the gate dielectric and other layers of the gateelectrode. In some implementations, the capping layer includes a metaland nitrogen, such as titanium nitride (TiN), tantalum nitride (TaN),tungsten nitride (W₂N), titanium silicon nitride (TiSiN), tantalumsilicon nitride (TaSiN), or combinations thereof. A work function layerincludes a conductive material tuned to have a desired work function(such as an n-type work function or a p-type work function), such asn-type work function materials and/or p-type work function materials.P-type work function materials include TiN, TaN, Ru, Mo, Al, WN, ZrSi₂,MoSi₂, TaSi₂, NiSi₂, WN, other p-type work function material, orcombinations thereof. N-type work function materials include Ti, Al, Ag,Mn, Zr, TiAl, TiAlC, TaC, TaCN, TaSiN, TaAl, TaAlC, TiAlN, other n-typework function material, or combinations thereof. A glue/barrier layercan include a material that promotes adhesion between adjacent layers,such as the work function layer and the metal fill layer, and/or amaterial that blocks and/or reduces diffusion between gate layers, suchas the work function layer and the metal fill layer. For example, theglue/barrier layer includes metal (for example, W, Al, Ta, Ti, Ni, Cu,Co, other suitable metal, or combinations thereof), metal oxides, metalnitrides (for example, TiN), or combinations thereof. A metal fill layercan include a suitable conductive material, such as Al, W, and/or Cu. Itis noted that the metal gate stack 234 is hidden behind the gate spacer230 when viewed along the X direction.

Referring now to FIGS. 1 and 11 , the method 100 includes a block 116Awhere a high pressure anneal (HPA) process 400 is performed to theworkpiece 200. In some embodiments, the HPA process 400 may be performedafter formation of the source/drain features 226, 228 and a metal gatestack 234 at block 114. In some other embodiments, the method 100 mayalternatively include a block 116B (rather than block 116A) where theHPA process 400 is performed after the block 118 (to be describedbelow). IC manufacturing process flow is typically divided into threecategories: front-end-of-line (FEOL), middle-end-of-line (MEOL), andback-end-of-line (BEOL). FEOL generally encompasses processes related tofabricating IC devices, such as transistors. For example, FEOL processescan include forming active regions, isolation features, gate structures,and source/drain features. MEOL generally encompasses processes relatedto fabricating contacts to conductive features (or conductive regions)of the IC devices, such as contacts to the gate structures and/or thesource/drain features. BEOL generally encompasses processes related tofabricating a multilayer interconnect (MLI) feature that interconnectsIC features fabricated in FEOL and MEOL processes. In view of the ICmanufacturing process flow described above, the PSA process 300 at block110 takes place at the FEOL level or is an FEOL process; the HPA process400 at block 116A takes place at the MEOL level or is an MEOL process;and the HPA process 400 at block 116B takes place at the BEOL level oris a BEOL process.

In either in the block 116A or the block 116B, the HPA process 400 maybe performed by means of convection heating at a second temperature (T2)between about 350° C. and about 450° C., at a second pressure (P2)between about 10 atm and about 20 atm, and in a gas ambient includinghydrogen. As compared to the parameters for the PSA process 300, thesecond temperature (T2) of the HPA process 400 is smaller than the firsttemperature (T1) of the PSA process 300, and the second pressure (P2) ofthe HPA process 400 is greater than the first pressure (P1) of the PSAprocess 300. The elevated second pressure (P2) is necessary to drivehydrogen from structure adjacent to channel regions into the channelregions. As compared to the PSA process 300, the second temperature (T2)of the HPA process 400 may not exceed 450° C. to avoid reflow or damagesto metal-containing structures formed at the BEOL or the MEOL levels.

Referring now to FIGS. 1 and 12 , the method 100 includes a block 118where further processes are performed. As described above, such furtherprocesses include formation of additional interlayer dielectric layers(ILDs) and contact features formed in the additional ILDs. Example ofthe contact features may include gate contacts to the metal gate stack(e.g., the metal gate stack 234 in FIG. 10 ), source/drain contacts 238and 240 to the source/drain features (e.g., the p-type source/drainfeature 226 and the n-type source/drain feature 228 in FIG. 10 ), gatecontact vias coupling gate contacts to overlying conductive layers,source/drain contact vias coupling source/drain contacts to overlyingconductive layers, or conductive rail structures connecting varioussource/drain/gate contacts. For ease of illustration, only source/draincontacts 238 and 240 are illustrated in FIG. 12 . In someimplementations, the contact feature may include a barrier layer, aliner and a metal fill layer. The barrier layer may be formed of Ta,TaN, TaC, Ti, TiN, TiC, and other suitable material that can blockoxygen diffusion. The liner may be formed of suitable metal, metalnitride, or metal carbide, such as Co, CoN and RuN. The metal fill layermay be formed of any suitable conductive material, such as W, Ni, Ta,Ti, Al, Cu, Co, TaN, TiN, Ru, and/or other suitable conductivematerials. The contact features are configured to route signals betweendevices of the semiconductor device 200 and/or distribute signals (forexample, clock signals, voltage signals, and/or ground signals) to thedevices of the semiconductor device 200.

In some implementations illustrated in FIG. 12 , a silicide layer 236may be formed on the recessed p-type source/drain features 226 andn-type source/drain feature 228. In some implementations, the silicidelayer 236 is formed by depositing a metal layer over the p-typesource/drain features 226 and n-type source/drain feature 228. The metallayer includes any material suitable for promoting silicide formation,such as nickel, platinum, palladium, vanadium, titanium, cobalt,tantalum, ytterbium, zirconium, other suitable metal, or combinationsthereof. The semiconductor device 200 is then heated (for example,subjected to an annealing process) to cause constituents of the p-typesource/drain features 226 and n-type source/drain feature 228 (forexample, silicon and/or germanium) to react with the metal. The silicidelayers thus include metal and a constituent of the p-type source/drainfeatures 226 and n-type source/drain feature 228 (for example, siliconand/or germanium). In some implementations, the silicide layer 236includes nickel silicide, titanium silicide, or cobalt silicide. Anyun-reacted metal, such as remaining portions of the metal layer, isselectively removed by any suitable process, such as an etching process.Operations of block 116B in FIG. 1 are illustrated in FIG. 12 . Asdescribed above, the HPA process 400 may be performed after operationsof block 118 are performed to form contact features that contain metal.

Methods of the present disclosure provide advantages. The PSA processperformed after formation of the silicon cap layer but before formationof metal gate stacks and contact features may include high annealtemperature to crystallize the silicon cap layer and to reduce DIT dueto formation of the silicon cap layer, without causing reflow or damagesto the metal gate stack and metal-containing contact features. The HPAprocess performed at the MEOL or the BEOL level may include highpressure to drive hydrogen into the channel regions and to reduce DITdue to processes performed after the PSA process. The two-stage DITreduction disclosed in the present disclosure suppresses DIT, reduceslow-field scattering, increases hole mobility, reduces sub-thresholdswing, and reduces channel resistance. In some embodiments, channelresistance in a p-type fully strained channel (such as a channel regionformed in the p-type fins 216) may be reduced by between about 5% andabout 10%, when PSA process is performed in addition to the HPA process.Experimental data prove that such reduction in channel resistance isindependent from channel length. In some instances, it is observed thatthe 5% to 10% reduction in channel resistance may approximately resultin between about 5% and about 10% of increase in hole mobility.

The present disclosure provides embodiments of methods of fabricatingsemiconductor devices. In one embodiment, a method is provided. Themethod includes forming, on a substrate, a first fin formed of a firstsemiconductor material and a second fin formed of a second semiconductormaterial different from the first semiconductor material, forming asemiconductor cap layer over the first fin and the second fin, andannealing the semiconductor cap layer at a first temperature while atleast a portion of the semiconductor cap layer is exposed.

In some embodiments, the first fin includes a p-type channel region, thesecond fin includes an n-type channel region, the first semiconductormaterial includes germanium, and the second semiconductor materialincludes silicon. In some implementations, the semiconductor capconsists essentially of silicon. In some instances, the firsttemperature is greater than 800° C. In some embodiments, the firsttemperature is between about 800° C. and about 1050° C. In someembodiments, the forming of the semiconductor cap layer includesdepositing silicon using atomic layer deposition (ALD). In someimplementations, the forming of the semiconductor cap layer includesepitaxially growing silicon over the first fin and the second fin. Insome examples, the annealing of the semiconductor cap layer increasescrystallinity of the semiconductor cap layer.

In another embodiment, a method is provided. The method includes Amethod includes forming, on a substrate, a first fin including siliconand germanium, forming, on the substrate, a second fin includingsilicon, forming a silicon cap layer over the first fin and the secondfin, performing a first anneal at a first temperature and a firstpressure while at least a portion of the silicon cap layer is exposed,forming source/drain features over source/drain regions of the first finand the second fin, forming a gate structure over channel regions of thefirst fin and the second fin, and after the forming of the gatestructure, performing a second anneal at a second temperature and asecond pressure while no portion of the silicon cap layer is exposed.

In some embodiments, the first temperature is greater than the secondtemperature and the second pressure is greater than the first pressure.In some implementations, a germanium content in the first fin is betweenabout 20% and about 60%. In some instances, the forming of the siliconcap layer includes depositing silicon using atomic layer deposition(ALD). In some embodiments, the forming of the silicon cap layerincludes epitaxially growing silicon over the first fin and the secondfin. In some implementations, the first temperature is between about800° C. and about 1050° C. and the first pressure is between about 0.01atmosphere (atm) and about 1.1 atm. In some embodiments, the secondtemperature is between about 350° C. and about 450° C. and the secondpressure is between about 10 atmosphere (atm) and about 20 atm. In someinstances, the gate structure includes a high-k dielectric layer, a workfunction layer, and a metal fill layer.

In still another embodiment, a method is provided. The method includesforming, on a substrate, a first fin including silicon and germanium,forming, on the substrate, a second fin including silicon, forming asilicon cap layer over the first fin and the second fin, and immediatelyafter the forming of the silicon cap layer, performing a first anneal ata temperature between about 800° C. and about 1050° C.

In some embodiments, the forming of the silicon cap layer includesdepositing silicon using atomic layer deposition (ALD). In someimplementations, the forming of the silicon cap layer includesepitaxially growing silicon over the first fin and the second fin. Insome instances, the method further includes forming a gate structureover channel regions of the first fin and the second fin, and after theforming of the gate structure, performing a second anneal at atemperature between about 350° C. and about 450° C.

The foregoing has outlined features of several embodiments. Thoseskilled in the art should appreciate that they may readily use thepresent disclosure as a basis for designing or modifying other processesand structures for carrying out the same purposes and/or achieving thesame advantages of the embodiments introduced herein. Those skilled inthe art should also realize that such equivalent constructions do notdepart from the spirit and scope of the present disclosure, and thatthey may make various changes, substitutions and alterations hereinwithout departing from the spirit and scope of the present disclosure.

What is claimed is:
 1. A method, comprising: forming, on a substrate, afirst fin formed of a first semiconductor material and a second finformed of a second semiconductor material different from the firstsemiconductor material; depositing a semiconductor nitride liner overthe substrate and sidewalls of the first fin and the second fin; formingan isolation feature over the semiconductor nitride liner and betweenthe first fin and the second fin; after the forming of the isolationfeature, epitaxially depositing a silicon cap layer along the sidewallsof the first fin and the second fin using a silicon-containing sourcegas; and annealing the silicon cap layer at a first temperature while atleast a portion of the silicon cap layer is exposed.
 2. The method ofclaim 1, wherein the first fin comprises a p-type channel region,wherein the second fin comprises an n-type channel region, wherein thefirst semiconductor material comprises germanium, wherein the secondsemiconductor material comprises silicon.
 3. The method of claim 1,wherein the epitaxially depositing of the silicon cap layer comprisesuse of silane (SiH4), silicon tetrachloride (SiC14), trichlorosilane(TCS), or dichlorosilane (SiH2C12).
 4. The method of claim 1, whereinthe first temperature is greater than 800° C.
 5. The method of claim 1,wherein the first temperature is between about 800° C. and about 1050°C.
 6. The method of claim 1, wherein the epitaxially depositing of thesilicon cap layer comprises depositing silicon using atomic layerdeposition (ALD).
 7. The method of claim 1, wherein the annealing of thesilicon cap layer increases crystallinity of the silicon cap layer. 8.The method of claim 1, wherein the forming of the isolation featurecomprises: blanketly depositing a dielectric layer over thesemiconductor nitride liner; planarizing the dielectric layer to exposethe first fin and the second fin; and etching back the dielectric layerto form the isolation feature such that a portion of the first fin and aportion of the second fin rise above the isolation feature.
 9. Themethod of claim 1, further comprising: after the annealing, depositingan interfacial layer over the silicon cap layer; and forming apolysilicon dummy gate stack over the interfacial layer, wherein theinterfacial layer comprises silicon oxide.
 10. A method, comprising:forming, on a substrate, a first fin comprising silicon and germanium;forming, on the substrate, a second fin comprising silicon; forming asilicon cap layer over the first fin and the second fin; performing afirst anneal at a first temperature and a first pressure while at leasta portion of the silicon cap layer is exposed; forming source/drainfeatures over source/drain regions of the first fin and the second fin;forming a gate structure over channel regions of the first fin and thesecond fin; and after the forming of the gate structure, performing asecond anneal at a second temperature and a second pressure while noportion of the silicon cap layer is exposed; wherein the firsttemperature is greater than the second temperature, wherein the secondpressure is greater than the first pressure.
 11. The method of claim 10,wherein a germanium content in the first fin is between about 20% andabout 60%.
 12. The method of claim 10, wherein the forming of thesilicon cap layer comprises depositing silicon using atomic layerdeposition (ALD).
 13. The method of claim 10, wherein the forming of thesilicon cap layer comprises epitaxially growing silicon over the firstfin and the second fin.
 14. The method of claim 10, wherein the firsttemperature is between about 800° C. and about 1050° C., wherein thefirst pressure is between about 0.01 atmosphere (atm) and about 1.1 atm.15. The method of claim 10, wherein the second temperature is betweenabout 350° C. and about 450° C., wherein the second pressure is betweenabout 10 atmosphere (atm) and about 20 atm.
 16. The method of claim 10,wherein the gate structure comprises a high-k dielectric layer, a workfunction layer, and a metal fill layer.
 17. A method, comprising:forming, on a substrate, a first fin comprising silicon and germanium;forming, on the substrate, a second fin comprising silicon; forming anisolation feature over the substrate and between the first fin and thesecond fin; after the forming of the isolation feature, forming asilicon cap layer over the first fin and the second fin; after theforming of the silicon cap layer, performing a first anneal at atemperature between about 800° C. and about 1050° C.; forming a gatestructure over channel regions of the first fin and the second fin; andafter the forming of the gate structure, performing a second anneal at atemperature between about 350° C. and about 450° C.
 18. The method ofclaim 17, wherein the forming of the silicon cap layer comprisesdepositing silicon using atomic layer deposition (ALD).
 19. The methodof claim 17, wherein the forming of the silicon cap layer comprisesepitaxially growing silicon over the first fin and the second fin. 20.The method of claim 8, wherein the forming of the isolation featurecomprises: after the etching back, selectively removing thesemiconductor nitride liner above the isolation feature.